The present invention relates broadly to electro-optical systems, in general, and more particularly, to an improvement which references the boresight axis of the electro-optical system with a predetermined frame reference position in the frames of electrical information generated by an electro-optical sensor included in the system.
In an electro-optical tracking system, very accurate position information related to a boresighted target is normally required for system operational effectiveness. The measuring system is generally based on a raster scanning electro-optical sensor where all position measurements are assumed relative to an actual boresight axis of the system. However, the measurement space is usually set up in a purely arbitrary manner, that is, the actual boresight axis may be in an arbitrary position in the raster scan of the electro-optical sensor. In order to obtain accurate position measurements, a way should be provided to center the true optical axis or boresight axis of the system in the raster scan of the electro-optical sensor.
Generally, for a given set of static conditions, the raster scan center of the electro-optical sensor may be aligned with the boresight axis of the system by setting bias values in the sensor's raster scan mechanism to introduce offsets which adjust the image of the actual boresight axis. Since most electro-optical tracking systems include complex combinations of optical elements, it is generally understood that many of the optical elements may incur movement. In fact, there are some tracking systems which utilize rotatable optical elements. Of course, any substantial movement of one or more of the optical elements from the static conditions used to set the bias values in the raster scan mechanism is likely to cause misalignment between the actual boresight axis and the raster scan center (apparent boresight axis). To make matters worse, optical elements are additionally vulnerable to thermal variations which may further compound any apparent misalignment problems causing the tracking reference measurements to be even more inaccurate.
Since the boresighting requirements of an electro-optical tracking system are normally extreme, it may be necessary, in some cases, to compensate for the relatively slow dynamic variations between the actual and apparent boresight axes. For example, in one known system, it is necessary to measure the actual boresight axis variations and introduce signals representative thereof in the apparent tracking position measurements for compensating purposes. However, because of the non-linearities created by the raster scan of the electro-optical sensor, there is no direct proportionality for the position difference in the raster scan between the actual and apparent boresight axes, i.e., the distance from the center of the raster scan where the axis image is actually projected. Consequently, any compensation attempt in this regard generally may still result in an inaccurate tracking position measurement.
Apparently, there is only one stable reference point in the raster scan of the electro-optical sensor for deriving position measurements and that is the raster scan center. Accordingly, a boresighting system which maintains the actual boresight axis at the center of the sensor's raster scan through the aforementioned slow moving dynamic variations of one or more of the optical elements thereof appears most desirable for achieving the required accuracy in deriving tracking measurements. A description of such a system is included in the specification heretofollow.